Human herpesvirus 6 (HHV6) is a DNA virus originally isolated from patients with lymphoproliferative disorders, harbingers of lymphoid malignancies. The oncogenic potential of HHV6 has been suggested by its ability to transform mouse and human cell lines in vitro, although epidemiologic and molecular evidence that supports the etiological role of HHV6 in malignancies remains controversial. The virus is thought to be transmitted via oral, perinatal, and sexual routes, but the possibility of genetic transmission of HHV6 was recently suggested by a report of chromosomal integration of HHV6 in a patient with acute lymphocytic leukemia (ALL) and her two family members (a son and a granddaughter). In order to expand on this observation and to characterize the chromosomal integration of HHV6, we are conducting a cross-sectional survey of HHV6 chromosomal integration among Japanese ALL patients and their first-degree family members, in collaboration with several medical schools and their teaching hospitals in Japan. Using a fluorescent in-situ hybridization (FISH) method, we will attempt to directly detect the presence of HHV6 in the chromosomes of the subjects. Successful confirmation of HHV6 transmission via genetic material will yield new hypotheses regarding the role of viruses in familial aggregation of cancers.